Agricultural, aviation, and commercial buildings have large side and end walls with openings to accommodate vehicles, equipment, aircraft and large machinery. Bi-fold doors hinged to side and end wall frames of the building are used to selectively open and close the openings. Bi-fold doors have upper and lower door panels hinged along adjacent sections with the upper panel hinged to the headers located above the openings. Manually operated winches have been used to open and close bi-fold doors. Winches driven with electric motors are also used to open and close bi-fold doors. The winches have cables, such as aircraft cable, and cylindrical drums which rotate to wind and unwind cables on and from the drums. The cables wind along the length of the drums which have longitudinal lengths to accommodate the cable turns. The upper ends of the cables are secured to the top of the upper door panel. The electric motors rotate the drums to wind the cables on the drums to open the bi-fold doors. When the drive of the electric motors is reversed, the cables are unwound from the drums to allow the bi-fold doors to move from the open positions to the closed positions. The bi-fold door opening and closing episodes occur at a constant speed determined by the speed of rotation of the winch drums. The cables require periodic adjustments and maintenance and are subject to wear which reduces their working life.